1. Field of the Invention
This invention relates to the production of feedback signals indicative of the rate of rotation of a system, such as a space craft for use in controlling a torque applied to the space craft with drive devices such as control moment gyros (CMGs).
2. Description of the Prior Art:
CMGs are well known in the art for use in controlling the torque applied to a space craft to control its orientation or rate of rotation with respect to reference axis or to another body such as earth. For example, a satellite in space may have its rate of rotation controlled about one or more axes (usually three) by fixing the gimbal frames of one or more CMGs (usually four to provide redundancy) to the frame of the spacecraft and applying a command rate to the gimbals of the CMGs which then create torques which are transferred to the frame of the spacecraft to obtain the desired rotation rate. In such systems it is desirable to provide closed loop feedback for better control by assuring that the actual rates are the same as the commanded rates. Such systems have employed tachometers to supply a signal indicative of the relative rate of rotation between the gimbal frame and the inner gimbal assembly. However, tachometers have greater size and weight than is desired, have low rate threshold (the signals can't be discerned from the noise), have variable rate ripple (voltage that varies without change in gimbal rate) and must be mounted colinearly with the gimbal. A better system has been described in the above referenced Foshage application where a pair of fiber optic gyros are employed, one to measure the absolute rate of rotation of the inner gimbal assembly and one to measure the absolute rate of rotation of the frame attached to the spacecraft. While this system provides excellent rate control, it requires the use of two separate absolute rate sensors (e.g. fiber optic gyros).